如何在C ++中实现交换 [英] How is swap implemented in c++

问题描述

我刚刚开始学习元编程，我想知道交换的实现. 谁能帮助我并解释元编程中的特征概念吗?谢谢.

I just start learning metaprogramming, I wonder the implementation of swap. Can anyone help me and explain the idea of traits in metaprogramming? thanks.

推荐答案

std::swap被实现为模板，因此它可以交换任何给定类型的两个变量的值.其原型如下:

std::swap from <algorithm> is implemented as a template, so that it can exchange the values of two variables of any given type. Its prototype looks like this:

template <typename T> void swap(T& a, T& b);

典型的实现是将一个值保存在临时变量中，将第二个值复制到第一个变量，然后将临时值放入第二个变量.在C ++中，这涉及复制构造函数和赋值运算符.

The typical implementation is to save one value in a temporary, copy the second value to the first variable, and then to put the temporary value into the second variable. In C++, this involves a copy constructor and assignment operators.

对于大型物体，所有这些构造和复制都可能非常昂贵.如此多的对象(包括大多数(全部)STL容器)都具有重载的实现，该实现通过交换一些指针值来工作.交换指针的速度非常快，并且可以避免任何潜在的故障(例如在复制构造函数中进行内存分配).重载的std::swap转发到成员函数(通常也称为swap).成员函数可以访问内部函数，因此可以使用更有效的方法.重载可能看起来像这样(简化):

For large objects, all of that construction and copying can be expensive. So many objects, including most (all?) of the STL containers have an overloaded implementation that works by exchanging a few pointer values. Exchanging pointers is very fast, and it avoids any potential failures (like memory allocation during the copy constructor). The overloaded std::swap forwards to a member function (often also called swap). The member function has access to the internals, so it can use a more efficient approach. The overload might look something like this (simplified):

template <typename T> void swap<vector<T> >(vector<T>& a, vector<T>& b) {
  a.swap(b);
}

如果要查看实际代码，可以浏览std头文件以查找编译器和操作系统.

If you want to see the real code, you can browse the std header files for your compiler and OS.

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